Optical waveguide structure

ABSTRACT

PCT No. PCT/GB97/00266 Sec. 371 Date May 4, 1998 Sec. 102(e) Date May 4, 1998 PCT Filed Jan. 30, 1997 PCT Pub. No. WO97/28481 PCT Pub. Date Aug. 7, 1997An optical fibre is formed with longitudinal recesses (11, 12) which extend toward its core (3) through cladding region (2). The recesses receive electrode structures (20a, b) made of glass, which include tongues (23a, b) that fit into the recesses, on which metal electrode strips (a, b) are formed. A potential difference applied between the strips (a, b) can induce electro-optic effects in the core.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an optical waveguide structure in which anelectric field may be applied to achieve electro-optic effects, and hasparticular application to optical fibres.

2. Related Art

It is well known that lithium niobate changes its opticalcharacteristics in response to an applied electric field and can be usedas an electro-optic modulator or a non-linear optical element,particularly in an optical fibre. However, it has a number of drawbacks,particularly high coupling losses when coupled to a standard fibre and alow-photorefractive damage threshold, which have prompted aninvestigation of the electro-optic effects of glassy materials and inparticular silica.

Thermally assisted poling of silica has been known to induceelectro-optic coefficients in both bulk silica and optical fibres, andreference is directed to L. Li & D. N. Payne "Permanently-Induced LinearElectro-Optic Effect in Silica Optical Fibres, Dig. Conf. Integrated andGuided Wave Optics, 1989 OSA, Paper TuAA2-1 (1989). However, thecoefficient induced in this way is not sufficiently high to allowpractical devices to be constructed.

It has recently been found that germanosilicate fibre, which isphotosensitive to u.v. light, can be photo-excited with incident u.v.radiation to produce an electro-optic coefficient comparable to that oflithium niobate. Reference is directed to T. Fujiwara, D. Wong, Y. Zhao,S. Fleming, V. Grishina & S. Poole, "UV-Excited Poling and ElectricallyTunable Bragg Gratings in a Germanosilicate Fibre", Postdeadline PaperOFC '95 (Feb '95). The u.v. technique has a significant furtheradvantage over thermal poling in that it permits the writing of gratingsand other structures in the fibre.

In order to achieve a sufficiently high applied field for the fibre, itis has previously been proposed to modify a conventional germanosilicatefibre which has a Ge doped core of relatively high refractive indexsurrounded by SiO₂ cladding of relatively low refractive index, so as toinclude longitudinal apertures in the cladding to receive electrodes inthe form of metal wires running generally parallel to the core onopposite sides. By placing the electrodes close to the core, within thecladding, a sufficiently high field can be developed across the core inorder to induce changes in the refractive index of the core. Referenceis directed to S. C. Fleming, T. Fujiwara and D. Wong "UV Excited Polingof Germanosilicate Fibre" OSA '95 Photosensitive non-linearity in Glasswaveguides - Fundamentals and Applications, OSA Technical Digest Vol. 221995. The fibre was fabricated by milling a pair of holes into the endface of a preform close to its core and positioned diametrically acrossthe core with respect to one another. The preform was then drawn intofibre in a conventional manner so as to form a fibre with a corediameter of 8 μm and a spacing of 18 μm between the apertures thatreceive the electrodes. The apertures were of a diameter of the order of70 μm and the electrodes wires had a diameter of the order of 50 μm. Theelectrode length was in one example 6 cm.

A disadvantage of this structure is that the electrodes need to beinserted into the fibre after formation. It will be seen that theelectrode wires are of very small diameter and consequently difficult tohandle. Furthermore, because the structures are so small, the electrodeshave to be arranged to extend out of the apertures at opposite ends ofthe device in order to avoid risk of them touching, which thereforerequires long connection leads. The entry of the leads in end faces ofthe fibre makes it very difficult to splice the fibre to conventionaloptical fibres, so that it cannot be included readily in opticalcircuits. Conventional fusion splicing could not be used because theheat required causes air in the holes to expand and distort or damagethe heat softened glass of the fibre. Also, the holes need to be of alarger diameter than the electrode wires to allow them to be fitted,with the result that they are not held at a fixed distance from the coreof the fibre. This can result in a non-uniform field being applied, inuse, along the fibre.

Partial removal of the cladding of a fibre has been proposed in U.S.Pat. No. 5,265,178 for the purpose of allowing a doped polymer to beplaced close to the optical fibre for modulation purposes, rather thanfor applying a field to the core. Planar structures with modulatingelectrodes have been proposed in "Low-loss Strain Induced OpticalWaveguides in Strontium Barium Niobate at 1.3 μm wavelength", J. M. Marxet al Appl. Phys. Letts. 66 (3) January 1995, pp 274-276.

SUMMARY OF THE INVENTION

The present invention provides an alternative, more robust structure forovercoming the aforesaid disadvantages of the prior art.

In accordance with the present invention, there is provided a fibreoptic waveguide structure comprising: an elongate waveguide bodyincluding a core and cladding around the core, the body having an outersurface that includes longitudinally extending first and second regions,the first region being closer to the core than the second region, andelectrode means on the first region to apply an electric field acrossthe core.

The outer surface of the body may include a longitudinal recess, withthe first region being disposed in the recess.

The electrode means may comprise an elongate electrode support whichextends into the recess and an electrically conductive region on thesupport extending along the length thereof. The support may include anelongate body member with an upstanding tongue that fits into therecess. The body may be made of glass and the conductive region maycomprise a metallic coating formed on the glass.

Thus, in accordance with the invention, the electrodes may be readilyfitted without the need to thread fine wires into apertures, whichgreatly simplifies manufacture.

Alternatively, the electrode means may comprise an electricallyconductive layer on the first region, such as a metallisation layer,formed by evaporation techniques. The recess can be used to form amasking effect for the deposition, so as to allow the metal to beconfigured selectively on the first region.

The structure according to the invention has particular application tofibre optic waveguides that include a core and a cladding made of silicaglass, wherein the core is doped with Ge or B, so as to render thestructure photosensitive to u.v. light. However, the invention has wideapplication to many other material systems.

The structure according to the invention nay be used with advantage as aphase modulator and can be made sufficiently small that a fibrestructure is provided that can operate in single mode transmission.

Waveguides according to the invention may be formed by drawing from apreform, and the invention includes a method of fabricating a waveguidestructure including: preparing a preform with material for forming awaveguide core surrounded by material for forming a waveguide cladding,the preform having an outer surface that includes first and secondregions, the first region being closer to the core material than thesecond region, drawing the preform so as to produce a fibre opticwaveguide with the same general cross sectional configuration as thepreform but of extended length and reduced transverse dimensions, withthe first and second regions running longitudinally of the lengththereof, and providing an electrode that extends longitudinally over thefirst region of the outer surface.

The invention also includes a preform configured for use in this method.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the invention may be more fully understood an embodimentthereof will now be described by way of contrast with a prior art deviceas described by Fleming et al supra, reference being had to theaccompanying drawings in which:

FIG. 1 is a cross-section through a prior art waveguide structure;

FIG. 2 is a longitudinal section of the device shown in FIG. 1;

FIG. 3 illustrates a preform for use in manufacturing a waveguidestructure in accordance with the invention;

FIG. 4 is a schematic perspective view of a short length of a waveguideand one of its associated electrode structures, in accordance with theinvention;

FIG. 5 is a photograph of a section through a sample of a waveguide,corresponding to the waveguide shown in FIG. 4;

FIG. 6 is a sectional view of the entire waveguide structure inaccordance with an example of the invention;

FIG. 7 illustrates a fibre in accordance with the invention fusionspliced to conventional optical fibre at both ends;

FIG. 8 illustrates an alternative way of depositing an electrode in therecesses of the fibre;

FIG. 9 illustrates a fibre of FIG. 8, after deposition of the electrodesand with the recesses filled with an insulating material;

FIG. 10 illustrates in cross section an alternative embodiment ofoptical fibre in accordance with the invention with a single generallyrectangular recess;

FIG. 11 illustrates an alternative fibre in accordance with theinvention, with a curved recess that includes a filamentary electrode;

FIG. 12 illustrates a cross section through a tape fibre in accordancewith the invention;

FIG. 13 illustrates a transverse cross section of another tape fibre inaccordance with the invention; and

FIG. 14 illustrates a cross section of a fibre in accordance with theinvention with two recesses that enter the fibre from the same side.

DETAILED DESCRIPTION

Referring to FIGS. 1 and 2, a prior art structure is shown, as disclosedby Fleming et al supra. A silica based fibre 1 which is shown intransverse cross section in FIG. 1, has a core region 3 with a corediameter of 8 μm surrounded by SiO₂ cladding region 2 with an outerdiameter of 125 μm. The cladding region 2 has a lower refractive indexthan the core 3 such that light is guided along the core in a mannerwell known per se The core also includes apertures 4, 5 which runlongitudinally of the fibre and are positioned diametrically opposite tothe core. The apertures have a diameter of 70 μm and receive electrodewires 7, 8 of diameter 50 μm. The waveguide structure is formed bydrawing from a preform of the same general shape as shown in FIG. 1, inwhich the apertures 4, 5 are formed by drilling prior to the drawingprocess.

The waveguide structure has the advantage that the electrodes are placedclose to the core. In one example, the hole spacing is 18 μm so that ahigh field strength of 100 V/μm can be achieved

However, there are a number of difficulties with this structure. Thesmall diameter of the electrodes means that they are very difficult tohandle mechanically and it is difficult to thread them into theapertures 4, 5. It would be desirable to have both electrodes extendfrom the same end of the structure but due to the small diameter, thereis a significant risk that the bare electrode wires would touch oneanother and produce a short circuit. For this reason, it is usual tohave the wires extend out of opposite ends as shown in FIG. 2. A typicallength for the structure is 6 cm with the result that the wires need tobe brought together externally of the structure to a voltage source,with the disadvantage that bare electrode wires need to extendexternally, which makes the device impractical. Also, because theelectrode wires need to fit into the holes, they need to be a loose fit,with the result that their spacing from the core can vary along thelength of the fibre. This can result in a non uniform electric fieldbeing applied across the core along the length of the fibre.Furthermore, because the electrode wires protrude from end faces of thefibre, it is difficult to splice the fibre with its protrudingelectrodes to conventional fibre. Fusion splicing would be problematic,due to the expansion of air in the holes 4,5 which would distort thesoft glass produced by the heating used for conventional fusionsplicing.

The present invention provides a solution to these problems.

Referring now to FIG. 3, this shows a preform from which a waveguide forthe structure according to the invention is made. The preform consistsof a generally cylindrical block of silica glass 1 which includes acentral Ge or Ge and B doped region 3 that defines material for awaveguide core of relatively high refractive index surrounded by region2 with a low dopant concentration, that provides material for asurrounding cladding of the eventual waveguide. The preform is milled soas to form opposed parallel planar surface regions 9, 10. Recess regions11, 12 extend from the planar surface regions 9, 10 towards the coreregion 3.

The preform is then drawn by conventional techniques to form an opticalfibre and a short length of it is shown in FIG. 4, referenced 13. It canbe seen that the fibre has the same general shape as the preformalthough its dimensions are much reduced as it is of extended length ascompared with the preform. When viewed in transverse cross section, thefibre has a relatively broad dimension b in a first direction and arelatively narrow dimension w in a second direction normal to the firstdirection. Two recesses 11, 12 of depth d extend from planar surfaceregions 9, 10 towards the core 3 along the length of the fibre, therecesses having a width e. The bottom of the recesses 14, 15 are spacedfrom the core by distances f₁, f₂ respectively. The planar surfaceregions 9, 10 are connected by curved cylindrical surface regions 16, 18that extend along the length of the fibre. An example of the dimensionsof the structure are given in the following table.

    ______________________________________                                        parameter                                                                            b       w        d     e     f.sub.1                                                                             f.sub.2                             ______________________________________                                        dimension                                                                            250 μm                                                                             100 μm                                                                              30 μm                                                                            26 μm                                                                            9 μm                                                                             15 μm                            ______________________________________                                    

The refractive index of the cladding material 2 in this example is 1.454and the difference δn between the refractive index of the core andcladding is 0.01. A photograph of a sample of the waveguide is shown inFIG. 5. From the foregoing it will be understood that the bottom of therecesses 11, 12 form first surface regions of the fibre optic waveguidebody and the remainder of the outer surface of the body, namely theplanar surface regions 9, 10 and the curved surface region 16, 17, formsecond surface regions which are disposed further away from the corethan the first surface regions.

This structure has the advantage that an electrode can be placed in eachof the recesses 11, 12, over the first surface regions, applied acrossthe core 3. Furthermore, the electrodes can be placed at non-equaldistances from the core, where f₁ ≠f₂ in order to produce anasymmetrical electric field, if desired.

A suitable form of electrode structure 20 is shown in FIG. 4 andcomprises an elongate support made of u.v. transparent glass, which hasbeen etched, milled, sawed or otherwise cut so as to form elongatesymmetrical channels 21, 22 that define an upstanding tongue 23 on whichis deposited a metallic coating 24. The metallic coating 24 may bedeposited using conventional photolithography techniques, prior to theformation of the channels 21, 22. The support 20 has a width p and aheight q with each of the channels 21, 22 being of a width r and depths. An example of the dimensional parameters is set out in the tablebelow.

    ______________________________________                                        parameter p        q          r     s                                         ______________________________________                                        dimension 5000 μm                                                                             5000 μm 113 μm                                                                           35 μm                                  ______________________________________                                    

It will be seen that the tongue 23 of the support 20 is dimensioned soas to fit into the recess 12.

A similar support is provided with a tongue that fits into recess 11 andthe resulting structure is shown in cross section in FIG. 6 in which theelectrode supports are referenced 20a and b respectively. It can be seenfrom FIG. 6 that the metallic layers 24a, b can readily be inserted intothe recesses 11, 12 so as to be disposed in close proximity with thecore and thereby enable an electric field to be developed across thecore to alter its optical characteristics. The metallic layers 24a, bcan be run at the ends of the supports 20a, b onto exterior surfaces25a, b over side edges of the supports so as to provide external contactpads for providing a convenient external connection.

It will be appreciated that assembly of the structure is relativelysimple as compared with the described prior because the component partsmerely need to be push ed together with no complex threading ofelectrodes as hitherto.

As previously described, the core 3 of the structure is photosensitiveto u.v. light. When doped with Ge or Ge and B, it is photosensitive toradiation with a wavelength of 244 nm. Consequently, if desired, arefractive index Bragg grating can be written into the core e.g. using aphase mask. Reference is directed to G. Meltz et al "Formation of BraggGratings in Optical Fibres by Transverse Holographic Method" Opt. Lett.Vol. 14, No. 15, 823 (1989). Furthermore, a poled structure can berecorded in the fibre, by recording a u.v. pattern with an electricfield applied between the electrodes formed by the layers 24a, b in themanner described in Fujiwar et al, supra. Further details of fibrepoling methods can be found in "Phase material second-harmonicgeneration by periodic poling of fused silica" R. Kashyap et al, Appl.Phys. Lett. 64(11), 14 March 1994 pp 1332-1334; "High second-ordernonlinearities in poled silicate fibres" P. G. Kazansky et al, OpticsLetters, 15 May 1994, Vol. 19, No. 10, pp 701-703 and "Electro-opticphase modulation in a silica channel waveguide", A. C. Liu et al, OpticsLetters, Vol. 19, No. 7, 1 April 1994, pp 466-468. The resultingrefractive index grating can then be tuned by the application of aelectric field which alters the refractive index of the core by applyinga voltage to the metallic layers 24a, b. Furthermore, the device can beused as a phase modulator when no Bragg grating is recorded. The appliedfield alters the refractive index of the core and thus introduces aphase shift for optical signals traveling along the fibre. This can beused with advantage in a Mach Zehnder interferometer or in other opticaldevices.

The structure according to the invention has the advantage that it canbe manufactured in long lengths e.g. 50 cm or more whereas in the priorart, it was difficult to construct devices longer than 10 cm.

The structure can readily be spliced to conventional optical waveguidesand FIG. 7 shows a length of the waveguide structure 30 constructed asdescribed with reference to FIG. 6, spliced to lengths 31, 32 of silicabased optical fibre. Fusion splices are formed at 33 and 34 byconventional techniques well known to those skilled in the art.Electrode wires 35, 36 are connected to the metallic layers a,b whichare run onto the surfaces a,b as described with reference to FIG. 6.Thus, the ends of the structure are free of electrode wires and can bejoined by fusion splicing or other conventional butt joining techniquesto optical fibres 31, 32. In FIG. 7 the wires 35, 36 are shown at thesame end of the waveguide structure, but they could be at opposite ends.

An alternative way forming the electrode structures is shown in FIGS. 8and 9. In this example, metallisation layers are formed directly withinthe recesses 11, 12. Referring to FIG. 8, gold is evaporated byconventional techniques, using a resistive heater 37 driven by anelectrical power source 38 in a vacuum chamber (not shown). Au vapourtravels in the direction of arrows 39 so as to become deposited on thefirst surface region, namely the bottom of recess 12. Similar depositionis carried out on the bottom of recess 11. A gold layer is notsignificantly deposited on the side walls of the recesses 11, 12 butdeposition does occur on the planar surfaces 9, 10. The Au layer onsurfaces 9, 10 is removed subsequently either by rubbing or by using aself-adhesive tape applied to the surfaces to lift off the gold. Thus,the recesses 11, 12 can be used to self-mask the deposited electrodes.

Thus, referring to FIG. 9, metallisation layers 40a, b formed of thedeposited Au material extend along the bottom of the recesses 11, 12 soas to provide the electrodes. Thereafter, the recesses may be filledwith a suitable material 41, such as an electrically insulating compoundsuch as silicon rubber so as to protect the fibre from dielectricbreakdown and flash-over. This configuration has the advantage thatseparate glass electrode structures as shown in FIG. 4 do not need to befired to the fibre. It will be understood that other metallisationlayers could be used, instead of Au. Also, non-metallic electricallyconductive materials may be used, such polycrystalline silicon.

Many different, specific designs of fibre fall within the scope of theinvention and a number of alternatives will now be described by way ofexample.

Referring to FIG. 10, an alternative fibre cross section in accordancewith the invention is shown, which includes a single rectangular recess42 containing a metallic deposited electrode 43, formed in the mannerdescribed with reference to FIGS. 8 and 9. A second electrode 44 isformed by a vapour deposition on a planar surface 45 on the oppositeside of the fibre 2 to the electrode 43. It will be understood that thefibre is drawn from a correspondingly shaped preform of large dimensionsin the manner previously described with reference to FIG. 3, 4 and 5.The edges of the second electrode 44 may be defied by selectivelyrubbing the curved surfaces 16, 17 of the fibre to remove any depositedmetallic material therefrom, so as to leave the materials selectively onthe surface 45. Alternatively, a suitable conventional masking techniquemay be used during the vapour deposition process.

Referring to FIG. 11, another version of the fibre is shown, which issimilar to that of FIG. 10, in which the lowermost electrode 44 isformed on a planar surface 45 in the manner previously described.However, recess 46 has a generally curved cross section and theelectrode 47 is formed by a filamentary element which is physicallyfitted within the recess 46. It will be understood that the fitting ofthe element 47 is much simpler than in the prior art because it is notnecessary actually to thread the element through an aperture; instead itis laid in the recess 46 from outside the fibre. Furthermore, since onlyone filamentary element 47 is used, the possibility of it touching theother electrode 44 is materially reduced

Referring to FIG. 12, the optical fibre is in the form of a tape ofcross sectional width dimension w=50 μm and a cross sectional breadthdimension b=1 mm. The tape fibre includes longitudinal recesses 47, 48that extend from the outer surface of the waveguide body towards thecore 3 of the fibre, on opposite sides thereof. The base of each of therecesses 47, 48 is coated with a metallic, deposited conductive layer49, 50 to form electrodes which allow an electric field to be appliedacross the core 3. As shown in FIG. 12, the core may be disposedasymmetrically between the electrodes 49, 50. A typical example of thediameter of the core is 8 μm.

An alternative arrangement is shown in FIG. 13, in which the tape fibrehas the same dimensions as shown in FIG. 12 but instead of providingrecesses, the electrodes comprise metallisation layers 51, 52 formed onthe exterior body of the waveguide, on first surface regions 53, 54 thatare closer to the core than opposed second regions 55, 56. It will beseen that the core 2 is disposed asymmetrically between the electrodes51, 52.

Referring to FIG. 14, another fibre optic waveguide cross section isshown, which includes a flat surface 45, corresponding to the surfaceshown in FIG. 11, together with first and second recesses 55, 56, whichextend from the same side of the fibre, in parallel, on opposite sidesof the core 3, into the cladding region 2.

It will be understood that all of the embodiments shown in FIGS. 10 to14 are formed by drawing from a preform of corresponding shape, in themanner generally described with reference to FIG. 3 and 4. The preformfor each of the embodiments of FIGS. 10 to 14 may be made of materialsas described with reference to FIG. 3 and the core dimension may begenerally similar also.

Many modifications and variations of the described inventive structureare possible. For example it would be possible to construct a deviceincluding two cores spaced apart in a common cladding, each having anassociated pair of recesses such as 11 and 12 shown in FIG. 6, and theelectrode supports 20 having more than one tongue so as to fit into therecesses for each of the cores.

Furthermore, whilst the device has been described in relation to the agermanosilicate fibre, the invention is not limited to these materialsand can be used with any suitable material that exhibits anelectro-optic characteristic.

What is claimed is:
 1. A fibre optic waveguide structure comprising:anelongate waveguide body including a core and cladding around the core,the body having an outer surface that includes a longitudinallyextending recess, and electrode means in the recess to apply an electricfield across the core.
 2. A structure as in claim 1 wherein theelectrode means includes an elongate electrode support in the recess,and an electrically conductive region on the support extending along thelength thereof.
 3. A structure as in claim 2 wherein the supportincludes an elongate body member with an upstanding tongue which fitsinto the recess along the length thereof.
 4. A structure as in claim 2wherein the support is made of glass and the conductive region comprisesa metallic coating formed on the glass.
 5. A structure as in claim 1including first and second of said recesses, and first and second ofsaid electrode means in the recesses respectively, the core beingdisposed between the electrode means.
 6. A structure as in claim 5wherein the outer surface has an external surface which includes aplanar surface regions which extend longitudinally of the waveguide, andgenerally cylindrical surface regions which extend between the opposedplanar regions, the recesses being disposed in the planar regionsrespectively so as to extend towards the core.
 7. A structure as inclaim 5 wherein the outer surface of the cladding includes longitudinalplanar surface regions curved generally cylindrical surface regionswhich extend between the opposed planar regions, the recesses beingformed in the planar regions respectively to as to extend towards thecore.
 8. A structure as in claim 1 wherein the waveguide, in transversecross section, has a relatively broad dimension (b) in a first directionand a relatively narrow dimension (w) in a second direction extendingtransversely of the first direction, and the recess extends inwardlyfrom the exterior of the cladding towards the core in said seconddirection.
 9. A structure as in claim 8 wherein said broad dimension (b)is about 250 μm, said narrow dimension (w) is about 100 μm, and therecess has a depth (d) of about 30 μm.
 10. A structure as in claim 1,coupled at an end thereof to another fibre optic waveguide.
 11. Astructure as in claim 10 wherein the coupling comprises a fusion splice.12. A structure as in claim 1 wherein the electrode means comprises anelectrically conductive layer in the recess.
 13. A structure as in claim1 wherein the waveguide is formed of silica glass that has been doped toprovide the core.
 14. A structure as in claim 13 wherein the core dopantincludes Ge and B.
 15. A structure as in claim 1 configured to operatephase modulator.
 16. A structure as in claim 1 operable in single modetransmission.
 17. A structure as in claim 1 including a grating in thefibre.
 18. An optical waveguide structure comprising:an elongategenerally cylindrical light guiding waveguide body having a longitudinalouter surface, that includes material which has been photo-excited to apoled condition so as to exhibit an electro-optic coefficient, andelectrode means for applying an electric field into the waveguide bodyto alter said electro-optic coefficient, wherein the longitudinal outersurface includes at least one longitudinal recess, and the electrodemeans is disposed in the recess to apply the electric field into thewaveguide body along the length thereof.
 19. A fibre optic waveguidestructure comprising:an elongate waveguide body including a core andcladding around the core, the body having an outer surface that includeslongitudinally extending first and second regions, the first regionbeing closer to the core than the second region, the body includingmaterial which has been photo-excited to a poled condition so as toexhibit an electro-optic coefficient, and electrode means on the firstregion to apply an electric field across the core.
 20. A method offabricating a waveguide structure including:preparing a preform withmaterial for forming a waveguide core surrounded by material for forminga waveguide cladding, the preform having an outer surface that includesa recess, drawing the preform so as to produce a fibre optic waveguidewith the same general cross sectional configuration as the preform butof extended length and reduced transverse dimensions, with the recessrunning longitudinally of the length thereof, and providing an electrodethat extends longitudinally in the recess to apply an electric field tothe core.
 21. A fibre optic waveguide made by a method as in claim 20.